Machines for processing material such as are described in U.S. Pat. Nos. 5,427,680 and 6,070,417 process material at the wall of a reentrant lumen and employ a wall-clearing shuttle which recirculates through the lumen to clear from the lumen wall material which deposits thereon. The present invention effects replacement of a shuttle being used in such a machine with a different shuttle without interruption of the processing by diverting an operating shuttle from its normal channel to a replacement channel where it is exchanged for a new shuttle which then circulates around the lumen.